Is an amount of calories your body burns in the rest state just to maintain the physiologic functions. For average sized humans it’s around 1600-1800 calories/day for women and men correspondingly. Check your BMR at http://www.bmi-calculator.net/bmr-calculator/ This number is not stable and varies considerably, there are devices that can measure it more precisely by measuring your oxygen consumption. BMR + 500 cal = average energy expenditure for a person with an average physical activity throughout the day.